Ok, so we now suspect there are 100 million (+/- factor of 10) planets in our galaxy which might be capable of hosting life. Now, what is the probably of finding life on any one of them? Depends on the probably of life (a) arising independently or (b) spreading from one planet to another. Both unknowns. But both things we can gain insight on by finding out, definitively, if life ever arose independently on Mars, or ever spread there from Earth. If we find that life arose independently on Mars (probably testable by comparing its DNA to Earth DNA) then I suspect the implication is that the galaxy is full of life.

Ok, so we now suspect there are 100 million+ planets in our galaxy which might be capable of hosting life. Now, what is the probably of finding life on any one of them? Depends on the probably of life (a) arising independently or (b) spreading from one planet to another. Both unknowns. But both things we can gain insight on by finding out, definitely, if life ever arose independently on Mars, or ever spread there from earth. If we find that life arose independently on Mars (probably testable by comparing its DNA to earth DNA) then I suspect that the implication is that the galaxy is full of life.

Why do they have to be tidally locked? Mercury is not Tidally locked to our sun. I don't think we understand enough about how things become tidally locked to make assumptions about far away worlds yet.

If I was a betting man smart money would be on unstable non inhabitable system.Not that I'm saying there's no life out there, its a very very big universe, just there's always more ways something can go 'wrong' than 'right'.

Why do they have to be tidally locked? Mercury is not Tidally locked to our sun. I don't think we understand enough about how things become tidally locked to make assumptions about far away worlds yet.

We sure are clueless. It's not like we have a nearby tidally locked system to observe...

Why do they have to be tidally locked? Mercury is not Tidally locked to our sun. I don't think we understand enough about how things become tidally locked to make assumptions about far away worlds yet.

Depends on how narrowly you define tidal locking. Mercury is locked into a 3:2 spin-orbit resonance (it rotates 3 times per 2 orbits of the sun). Mercury isn't tidally locked only if you define tidal locking as strictly a 1:1 ratio.

Why do they have to be tidally locked? Mercury is not Tidally locked to our sun. I don't think we understand enough about how things become tidally locked to make assumptions about far away worlds yet.

Our understanding of how things work out there is next to nothing. We are basing everything on our own system which is silly. While it's all we have people need to acknowledge that many of our assumptions are likely wrong. We are still in the cradle stage of our development.

“Further observations, along with some refined statistical methods, now indicate that there are likely to be at least six planets in the system (and possibly a seventh), all packed in a region that's about half the distance from the Earth to the Sun”

How geologically stable are the planets going to be with 6 orbiting in a range of ½ AU, all with orbits of a month or two (IIRC from another article I read)

Why do they have to be tidally locked? Mercury is not Tidally locked to our sun. I don't think we understand enough about how things become tidally locked to make assumptions about far away worlds yet.

Depends on how narrowly you define tidal locking. Mercury is locked into a 3:2 spin-orbit resonance (it rotates 3 times per 2 orbits of the sun). Mercury isn't tidally locked only if you define tidal locking as strictly a 1:1 ratio.

Well, the concern about tidal locking and habitability is the creation of a hot spot/cold spot scenario where one side of the planet is in perpetual daylight, while the other is in perpetual night.

Mercury's year is 88 earth days. That means it gets a little more than 6 day/night cycles per earth year. In a habitability sense that's enough of a cycle to prevent the environmental issues a truly tidally-locked earth analogue would have.

We have found hundreds of planets so far using transiting and gravitational interactions with the parent star. Imagine what we will learn when we are able to detect atmospheric spectra or directly image these potentially habitable worlds. We are not even able to detect Earth sized worlds with current methods. I imagine we will find that there are thousands more potentially habitable small rocky planets or moons out there than we realize.

So let's strap a bunch of probes to some sort of nuclear propulsion device (pulse, fusion rocket, etc) and send it on its way. At a theoretical speed of .10c, the ship gets there in 200-250 years, counting for acceleration. 22 years later we start getting info from the system.

Sure, we might have better ways of getting there in 300 years, but that's never stopped previous explorers. And think of what we'll learn about the cosmos by sending a ship through interstellar space.

I guess the word "habitable" applies just to microbes, since their masses range from 2.7 to 3.8 that of earth. I don't know of anyone who could inhabit a planet with 3 Gs pulling at you 24/7.

Can't jump to that conclusion without knowing planet radius. If the planet is less dense that Earth, the mass may have a much larger radius, offsetting the impact of the larger mass on the local gravitational acceleration. If the planet is more dense than Earth, the local gravity would be higher than the 2.7-3.8 range.

What immediately jumped at me when reading the Habitable Exoplanets Catalog news release was that the small star packs nearly 5 planets in its narrow habitable zone (HZ). When we can find small planets around larger stars, we can expect to see up to 4-5 planets in the HZ.

Seems obvious (to me at least) that -c won't be habitable. In order to have a thick enough atmosphere to be habitable, and to have the right components in the atmosphere to become habitable after formation, it would have too thick an atmosphere at that mass. That would lead to the planet being a Super Venus, not a Super Earth.

OTOH, -e is VERY likely to be habitable; the extra mass would keep additional Green House gases during the early hot-phase, which would potentially be a self perpetuating process; the extra mass helps, there.

If we were to switch Mars and Venus in our solar system, it has been said, we'd likely have three habitable worlds. Bigger is better further out, not so good close.

Also, I agree that the naming convention is getting ridiculous, but it STARTED ridiculous.

Stars should be CAPITALS.... planets should be lowercase. And therefore, the first planet discovered around GJ 667C should have been GJ 667Ca, not GJ 667Cb.

I look forward to the day, in the future, where planets start getting actual names... I'm hoping they get named after their letter, so GJ 667Cf, for example, might end up being named "Flora" or something F.

what is the probably of finding life on any one of them? Depends on the probably of life (a) arising independently or (b) spreading from one planet to another. Both unknowns.

In fact, we know a) fairly well because life originated so early on Earth. Earliest fossils are now (arguably) ~ 3.8 Ga bp, with the resurrection of the Isua BIFs through microanalysis.

The reason is that we can model it as a stochastic process, and we need only one sample to constrain (test) its parameters. You have to bend over backwards and posit _different_ abiogenesis processes on Earth and Mars to reject the model, and that means claiming the fastest, apriori most likeliest process isn't - likeliest.

Also, this year saw Lane & Martin find a homology between alkaline hydrothermal vent chemistry and early autotroph (methanogene and acetogene) metabolism. How likely is it that a terrestrial doesn't exhibit alkaline hydrothermal vents? Not much.

Small question mark:

Quote:

If we find that life arose independently on Mars (probably testable by comparing its DNA to Earth DNA)

We know our cells started out with RNA metabolism, and we have good reasons why that is for various chemical reasons. (RNA can also act enzymatically, it is related to ATP energy currency which in turn is related to inorganic triphosphate, et many cetera.)

I'm not so sure though that DNA is the only possible metabolite that can act as an alternative genetic material (which was easier to replicate more faithfully). But I'm no biochemist.

Does the radiation frequency of the star limit the habitability? I'm under the impression that red dwarves mostly put out infrared, which would limit the types of earth organisms that could use it.

There are now at least two chlorophylls found that works in the near infrared region (NIR) suitable for most M dwarfs blackbody spectra (i.e. before atmosphere), Chl-d & f.

Chl-d, which has so far been found in at least 1 cyanobacteria, results in classic water splitting oxygenating photosynthesis under NIR conditions. (And in fact, that cyanobacteria is most productive there.)

EDIT: In an earlier version I claimed that they have found oxygenating photosynthesizers that live off of hydrothermal vent glows at 2 km or more depths. I forgot to check that.

In any case they wouldn't be prodigious photosynthesizers. But at least Chl-d hints at the potential for oxygenated oceans on M stars habitables and in ice moons.

Why do they have to be tidally locked? Mercury is not Tidally locked to our sun. I don't think we understand enough about how things become tidally locked to make assumptions about far away worlds yet.

Mercury is on the most eccentric orbit of planets, which is why it is a) in a temporary resonance, b) is chaotically changing orbits over time. It is pulled that way by other planets. [Wikipedia]

These planets have much more circularized and, as described, stable orbits.

Mind, tidal lock isn't much of a problem AFAIK. Even Venus, that is our best habitable zone tidal lock proxy due to its slow rotation (probably caused by lock tendencies AFAIU), redistributes heat. And it does so with pressures and surface winds that complex multicellulars can cope with. (IIRC some 50 km/h, even humans can cope even if we wouldn't like it. You would have to be a deep sea fish to cope with the pressure equivalent of a 1 km deep ocean.)

All you need is a denser atmosphere, which for the outermost Gliese 667C habitable would be better for habitability anyway.

So it cuts down the number of habitables, but isn't the showstopper some seem to think.

Does the radiation frequency of the star limit the habitability? I'm under the impression that red dwarves mostly put out infrared, which would limit the types of earth organisms that could use it.

In most of the 'habitability' models, people are just looking for earth sized, approximately earth distance, approximately solar type stars. There are other things like atmosphere that are really important as you can imagine. Measuring atmospheres is super duper hard right now, and most of the data is pretty garbage.

So yes, the type of star does limit whether the system can be habitable in practice. As does it's dynamics, etc. Too small a star and you need to be practically on top of it to stay warm, too big and you get too much radiation.

Gliese 667C f for example has likely mass M = 3.1, R = 1.5 according to the HEC. Both gives g ~ 1.5 times Earth gravity. [Be aware, the seeming consistency happens because they have assumed the same density too. But we, and they, are interested in Earth similar planets.]

The largest of these have g ~ 1.8 IIRC.

I should add that bacteria cell walls can cope with 40 atmospheres osmotic pressure differentials, meaning the same g differential. But beyond that, we find cellular life forms, even complex such, down to ~ 10 km depths in the oceans, or ~ 1000 atmospheres absolute pressure.

Similarly, mantis shrimps can generate 1000's of g of acceleration by spring loading exoskeletons. So they would be able to move in 1000's of g's.

Why do they have to be tidally locked? Mercury is not Tidally locked to our sun. I don't think we understand enough about how things become tidally locked to make assumptions about far away worlds yet.

?

Tidal locking happens pretty naturally to small objects near more massive ones in both theory and simulation.

--edit--For the more curious, the most commonly referenced text on planetary dynamics is probably'Solar System Dynamics', Murray & Dermott, 1999, Cambridge University Press

Also, if you don't want to pay anything, a wonderful free dynamics book by Alessandro Morbidelli (one of the authors of the 'Nice' model) is available on his web page:http://www.oca.eu/morby/celmech.pdfsubstitute .ps for pdf if you want a postscript (and you know you do).

What immediately jumped at me when reading the Habitable Exoplanets Catalog news release was that the small star packs nearly 5 planets in its narrow habitable zone (HZ). When we can find small planets around larger stars, we can expect to see up to 4-5 planets in the HZ.

It sounded more like there was one in the middle of the zone and one each on the inner and outer edges. That's pretty much what we have here in the Sol system. Earth is in the middle (though a bit close to the back edge), Mars is just on the outer border (would be greatly helped if it was the size of Earth or had Venus' atmosphere), and Venus is on the inner edge and in theory could be made habitable if you spun it up and scrubbed the CO2 from its atmosphere.

What immediately jumped at me when reading the Habitable Exoplanets Catalog news release was that the small star packs nearly 5 planets in its narrow habitable zone (HZ). When we can find small planets around larger stars, we can expect to see up to 4-5 planets in the HZ.

It sounded more like there was one in the middle of the zone and one each on the inner and outer edges. That's pretty much what we have here in the Sol system. Earth is in the middle (though a bit close to the back edge), Mars is just on the outer border (would be greatly helped if it was the size of Earth or had Venus' atmosphere), and Venus is on the inner edge and in theory could be made habitable if you spun it up and scrubbed the CO2 from its atmosphere.

Agreed, but their figure shows the question mark h and d as (more eccentric but stable) orbits close to the HZ.

I was interested purely in the potential of extremes (of 4-5 habitables/HZ), what we can see and not what we are most likeliest to see.

Most likeliest to see is 0-3 habitables/HZ, according to HEC stats (and other finds IIRC). FWIW, Mars was once habitable per Curiosity's recent finds, and Venus could and even should have been initially, so we had 2-3 habitables ourselves.

We don't know how systems looks farther out, and the behavior of migratory giants. According to the Nice model our own system's habitable zone has been affected by migration, which makes it even harder to compare.

Naming is hard. How about using mean AU as a postfix instead of a letter? Or even alongside the letter to preserve the order of discovery?

Mean AU is usually calculated based on other observables and assumptions that provide the star's mass, so later results can significantly change the mean AU value, forcing the name to change. The orbital period is an equivalent indicator of distance/ordering that is far less likely to change after discovery.

Whether the unit is time or distance based, there are steps to take that can also help the naming process. We can round to a whole number in some appropriately sized unit to guard against minor corrections and we can choose a unit size (which, for time, need not be conventional calendar units like days or years) to keep the range of values from becoming unwieldy. The only real unit size requirement though is that the rounding can't cause two objects in distinct, stable orbits in the same plane to get the same value/name.

Namespace collisions then are clearly special circumstances which can get special naming conventions as needed.

One minor negative in all of this is that the decision to name planets in order of their discovery is starting to create some very confusing exosolar systems. Assuming planet seven is confirmed at GJ 667C, then the order of planets will end up being b, h, c, f, e, d, and g, with g being furthest from the star. All of which makes keeping track of which planet is where rather challenging.

Ion propulsion and a swarm of nanoprobes should be able to do better than .10c after a few years of getting up to speed. I think. English major here -- is there a rocket scientist in the house?

Nuclear pulse can go much higher, but it is a question of how much mass you are accelerating. The more fuel (in this case, nuclear devices) the ship has to carry, the higher the mass and therefore requires more energy to accelerate. From what I've seen, around 10-12% would be optimal for a ship of reasonable mass.

What immediately jumped at me when reading the Habitable Exoplanets Catalog news release was that the small star packs nearly 5 planets in its narrow habitable zone (HZ). When we can find small planets around larger stars, we can expect to see up to 4-5 planets in the HZ.

It sounded more like there was one in the middle of the zone and one each on the inner and outer edges. That's pretty much what we have here in the Sol system. Earth is in the middle (though a bit close to the back edge), Mars is just on the outer border (would be greatly helped if it was the size of Earth or had Venus' atmosphere), and Venus is on the inner edge and in theory could be made habitable if you spun it up and scrubbed the CO2 from its atmosphere.

You'd have to supply an AWFUL lot of water to Venus to get it habitable; the water broken down eons ago, and the hydrogen escaped, so you can't reconstitute it no matter how much CO2 you scrub out. That's one of the primary reasons why I stated that -c was almost certainly uninhabitable; it's LARGE and on the inner edge, where the thicker atmosphere that you would expect the planet to retain would hold too much green house gasses, resulting in too much water getting into the atmosphere. Smaller planets, with less atmosphere, would almost certainly be cooler at the same distance, and thus more likely to retain the water on the inner edge; larger planets are better (due to green house gas build up) on the outer edge.

Ion propulsion and a swarm of nanoprobes should be able to do better than .10c after a few years of getting up to speed. I think. English major here -- is there a rocket scientist in the house?

I think the engineering challenge isn't "how fast can we make it go" inasmuch as "how fast can we make it go before it becomes impossible to stop without crushing it under G forces?"

Not a rocket scientist, though.

You stop it under the same limitations that you got it going, at an acceleration rate somewhere around one or at most a couple of G. This is why interstellar travel will be so damn inconvenient no matter how big your engines are you can only accelerate at a rate it can survive and then when its only pretty much just half way there and its time to turn around and start hitting the brakes. Just hope no massive and belligerent space moose stumbles into your path in the mean time.